1. Field of the Invention
The present invention relates to thin-film resistors and, more particularly, to a high-resistance thin-film resistor and a method of forming the resistor.
2. Description of the Related Art
A thin-film resistor is a semiconductor resistor that, as the name implies, is formed from a thin film of conducting resistive material. As with conventionally-formed discrete resistors, thin-film resistors are formed to provide a predefined resistance to the flow of current through the resistor.
FIGS. 1A-1B to 7A-7B show a series of views that illustrate a prior-art method 100 of forming a thin-film resistor that provides a predefined resistance. FIGS. 1A-7A show a series of plan views, while FIGS. 1B-7B show a series of cross-sectional views taken along lines 1B-1B through 7B-7B of FIGS. 1A-7A.
As shown in FIGS. 1A-1B, method 100 utilizes a conventionally-formed layer of insulation material 110, and begins with the sputter deposition of a thin layer of resistor material 112 on insulation layer 110. The thin layer of resistor material 112 can be implemented with, for example, a layer of titanium nitride (TiN) or a layer of tantalum nitride (TaN).
After the thin layer of resistor material 112 has been deposited, a patterned photoresist layer 114 is formed on the thin layer of resistor material 112. Patterned photoresist layer 114 is formed in a conventional manner, which includes depositing a layer of photoresist, projecting a light through a patterned black/clear glass plate known as a mask to form a patterned image on the layer of photoresist, and removing the imaged photoresist regions, which were softened by exposure to the light.
As shown in FIGS. 2A-2B, after patterned photoresist layer 114 has been formed, the exposed areas of the thin layer of resistor material 112 are etched to form a substantially-planar thin-film resistor structure 116 that touches the top surface of insulation layer 110. Once the etch has been completed, patterned photoresist layer 114 is removed in a conventional fashion, such as with an ash process.
Next, as shown in FIGS. 3A-3B, after patterned photoresist layer 114 has been removed, a layer of isolation material 120 is conventionally deposited using, for example, chemical vapor deposition. Following this, a patterned photoresist layer 122 is conventionally formed on isolation layer 120.
As shown in FIGS. 4A-4B, once patterned photoresist layer 122 has been formed, the exposed regions of isolation layer 120 are etched to form a pair of spaced-apart openings 124 in isolation layer 120 that expose a pair of spaced-apart surface regions 126 on the top surface of thin-film resistor structure 116. Once the pair of spaced-apart surface regions 126 have been exposed, patterned photoresist layer 122 is removed in a conventional manner.
As shown in FIGS. 5A-5B, following the removal of patterned photoresist layer 122, a layer of contact metal 130 is deposited in a conventional fashion to fill up the pair of openings 124 and cover the top surface of isolation layer 120. After contact metal layer 130 has been formed, a patterned photoresist layer 132 is conventionally formed on contact metal layer 130.
As shown in FIGS. 6A-6B, after patterned photoresist layer 132 has been formed, the exposed area of contact metal layer 130 is etched to form a pair of spaced-apart metal head contacts 134 that extend through isolation layer 120 to touch, and make electrical connections to, the pair of spaced-apart surface regions 126 on the top surface of thin-film resistor structure 116.
As shown in FIGS. 7A-7B, once the etch has been completed, patterned photoresist layer 132 is removed in a conventional fashion to form a thin-film resistor 140. As further shown in FIGS. 7A-7B, thin-film resistor 140 occupies an area on the top surface of insulation material 110, which in large part is defined by the length L times the width W of thin-film resistor structure 116.
One of the drawbacks of thin-film resistor 140 is that it is difficult to increase the resistance of thin-film resistor 140. One common approach to increasing the resistance of thin-film resistor 140 is to use a different higher-resistance material when depositing the thin layer of resistor material 112. Exotic materials that have high resistances, however, are often difficult or expensive to use in a conventional fabrication process.
Another common approach to increasing the resistance of thin-film resistor 140 is to increase the length of resistor 140 (and the spacing that lies between the metal head contacts 134). Significantly increasing the length of thin-film resistor 140, however, significantly increases the surface area that thin-film resistor 140 consumes which, in turn, increases the size and cost of a die that includes thin-film resistor 140.
Thus, there is a need for an approach to increasing the resistance of a thin-film resistor.